1. Field of the Invention
The present invention relates to the field of vertical diodes intended for rectification and/or protection used, for example, in power supplies of portable devices such as photographic cameras, shooting cameras, electronic agendas, wireless and/or mobile telephones, or portable computers.
2. Discussion of the Related Art
In such applications, Schottky diodes or PN junction diodes are in particular used.
FIG. 1A illustrates, in partial simplified cross-section view, a vertical power diode of Schottky type. The Schottky diode is formed in a lightly-doped semiconductor layer 1 of a first conductivity type, generally N. Layer 1 rests upon an N-type substrate 2 more heavily doped than layer 1. Substrate 2 is covered at its rear surface with a cathode metallization (not shown). At its front surface, layer 1 is partially covered with a thin layer 3 of a material capable of forming a Schottky junction with layer 1. Layer 3 forms one piece with an anode metallization 4. The diode generally comprises a guard ring 6 formed by front surface diffusion at the surface of layer 1. It should be noted that guard ring 6 can be considered as forming a PN diode with layer 1, but that this PN diode is not optimized for a protection function.
FIG. 1B illustrates the shape of characteristic I(V) of a Schottky diode. A Schottky diode exhibits a small forward voltage drop VF, generally smaller than 0.5 V, even for significant currents, that is, currents greater than 1 A. However, a Schottky diode has a poor behavior in reverse biasing, that is, the avalanche is unclear and reverse curve I(V) has a small slope, which results in that the voltage drop across the diode strongly increases when the reverse current flowing therethrough increases.
FIG. 2A is a simplified cross-section view of a vertical PN diode intended to ensure a protection function. The diode is formed in an N-type doped semiconductor substrate 11. At the rear surface, the diode comprises a heavily-doped N layer (N+) coated with a cathode metallization (not shown). At the front surface, substrate 11 comprises a lightly-doped P-type region 12 generally surrounded with a very heavily-doped P-type ring-shaped region 13. A portion of region 12 is solid with an anode metallization 14.
As illustrated in FIG. 2B, a PN diode exhibits in reverse biasing a satisfactory avalanche behavior, that is, beyond an avalanche voltage VBR, the slope of curve I(V) is very strong. However, in forward biasing, component 10 exhibits a relatively high voltage drop which varies along with the current. The doping of substrate 11 currently is on the order of from 1017 to 1018 at.cm−3 to optimize the reverse breakdown characteristic.
Thus, in applications where the rectifying function, that is, the obtaining of a small forward voltage drop, is essential, Schottky-type diodes are preferably used. However, in applications where the protection function, that is, the obtaining of a substantially constant reverse voltage, is essential, PN-type diodes are preferably used.
However, in many application among which are supply converters and/or supply or charge protection in portable devices, it is desirable to optimize both the rectifying function and the protection function. Since the only way to obtain a small forward voltage drop, much smaller than 0.6 V, is to use a Schottky diode, such greatly oversized diodes have to be used to avoid that the reverse voltage excessively increases along with the reverse current increase, which inevitably results in that other parameters, and especially the leakage current, are altered. It is impossible, whit Schottky diodes as well as with PN-junction components, to improve the reverse of forward current behavior without disadvantageously affecting their forward or reverse, diode, respectively. Indeed, any doping modification modifies both behaviors.